This grant proposal is submitted in response to the INTEGRATED PRECLINICAL/CLINICAL AIDs VACCINE DEVELOPMENT program [PAR-97-056]. It has been shown repeatedly that infection with attenuated SIV protects monkeys from challenge with virulent SIV. Although live-attenuated HIV vaccines may never be safe enough for clinical use, understanding the mechanism by which attenuated viruses confer protection will provide critical information for use in designing safe and effective HIV vaccines. We have recently demonstrated that animals immunized by vaginal inoculation with attenuated-SHIV 89.6 are protected from vaginal challenge with virulent SIVmac, Appendix A. Animals that had been infected with the attenuated SHIV for more than one year had antiviral cytotoxic T lymphocytes (CTL) and were protected from challenge with virulent SHIV, while those that were infected with the attenuated SHIV for approximately six months did not have detectable CTL and were riot protected. This association between the presence of CTL and the ability to resist superinfection with pathogenic SIV may partially explain the efficacy of attenuated viruses as vaccine in the SIV model, but it is unlikely to be the complete explanation. Although there are numerous questions, including safety, breadth of protection and route of administration, which need to be addressed in regard to attenuated virus vaccines, Project 1 will test two related hypotheses: 1) The type of cells and the anatomic distribution of cells infected with attenuated SHIV 89.6 changes between six and twelve months of infection. 2) This shift in the distribution of the attenuated virus infection produces a fundamental change in the nature of the antiviral immune response. The change in the immune response is manifested by altered helper T and effector T cell responses and by the generation of an environment within lymphoid tissues which is hostile to virus replication. This hostile environment is due to increased expression of antiviral cytokines (IFN- a, IFN-b, IFN-g and TNF) and chemokines (MIP1a, MIPlb and Rantes). We recognize that attenuated virus vaccines may not be acceptable for clinical use and we will use other vaccines to assess and compare the immune responses of those and attenuated virus. Project will test the ability of DNA vaccines to elicit protection from vaginal SIV challenge. We have chosen to focus on DNA immunization because the intracellular expression of the viral genes most closely resembles an attenuated virus infection and because naked DNA may offer a method for delivering an attenuated viral vaccine.